Supramolecular structures and process for making the same

ABSTRACT

The present invention provides a supramolecular structure produced by the process comprising reacting a multi-generation dendrimer with a monomer, where the dendrimer comprises a core molecule, a plurality of interior generations spherically disposed around the core molecule and an outermost generation comprising a plurality of dendritic branches having terminal groups sufficiently reactive to undergo addition or substitution reactions, where the monomer introduces a labile bond and at least one cross-linkable moiety to the terminal groups of each dendritic branch, and where the cross-linkable moiety is bonded to the terminal group via the labile bond; crosslinking the cross-linkable moieties of adjacent dendritic branches; and cleaving the labile bonds, thereby freeing the dendrimer and forming a molecule encapsulated within a cross-linked shell molecule. The present invention further provides a process for the production of the supramolecular structures.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 09/299,887,filed Apr. 27, 1999, which claims the benefit under 35 U.S.C. § 119(e)of U.S. Provisional Application No. 60/083,215, filed Apr. 27, 1998.

TECHNICAL FIELD OF THE INVENTION

The present invention is generally directed toward molecular structuresthat are synthesized by cross-linking the peripheral surface ofdendritic molecules. These cross-linked molecules are believed to bespherical in geometry or structure. The severing or cleaving of labilebonds within the dendritic molecule results in a spherical, cross-linkedshell molecule that can encapsulate or entrap smaller molecules.Accordingly, the present invention is directed toward these sphericalshell molecules, including those structures that contain smallermolecules within their structure, as well as a process for making thespherical shell molecules.

BACKGROUND OF THE INVENTION

Dendrimers are highly branched macromolecules formed by successivereactions of polyfunctional monomers around a core. Accordingly, asdendrimers grow from their core, their number of branches and terminalend groups increase, thereby increasing the density of these moleculesat their peripheral surface.

Dendrimers are distinguishable from polymers because they arenon-linear, hyper-branched structures that are synthesized in iterativefashion. The monomers from which they are constructed can generally bedefined as AB_(n) monomers, where n is usually 2 or 3, rather than thestandard AB monomers, which produce linear polymers. Accordingly, eachiteration of step-wise synthetic growth generally requires twice thenumber of monomers used in the previous iteration in the case of an AB₂monomer, or three times the number of monomers in the case of an AB₃monomer. The layer of monomers added in each iteration is called ageneration. The ultimate generation, or that generation farthest fromthe core, produces the periphery of the molecule.

Because dendrimers are produced in iterative fashion, they can besynthesized to very high molecular weight molecules with narrowmolecular weight distributions. Moreover, the nature of each generationcan be controlled by controlling the type of monomer employed includingthe periphery or end groups.

Based on the foregoing, and the relative novelty of dendriticstructures, the present invention furthers the art by makingadvancements in the modification of dendrimer structures, especially intheir ability to act as, or produce, a shell to entrap smallermolecules.

SUMMARY OF INVENTION

It is therefore an object of the present invention to provide a coredendrimer having a cross-linked peripheral surface.

It is another object of the present invention to provide a moleculewithin another cross-linked shell structure, where the two molecules arenot covalently or ionically bonded to each other.

It is another object of the present invention to provide a cross-linkedhollow shell structure.

It is another object of the present invention to provide asupramolecular structure that allows the flow of small molecules intoand out of the central core.

It is another object of the present invention to provide asupramolecular structure capable of entrapping smaller molecules.

It is another object of the present invention to provide asupramolecular structure having a cross-linked surface or peripheralporosity sufficient to entrap smaller molecules.

At least one or more of the foregoing objects, together with theadvantages thereof over the known art relating to dendritic moleculesand supramolecular structures, which shall become apparent from thespecification that follows, are accomplished by the invention ashereinafter described and claimed.

The present invention therefore provides a supramolecular structureproduced by the process comprising: reacting a multi-generationdendrimer with a monomer, where the dendrimer comprises a core molecule,a plurality of interior generations spherically disposed around the coremolecule and an outermost generation comprising a plurality of dendriticbranches having terminal groups sufficiently reactive to undergoaddition or substitution reactions, where the monomer introduces alabile bond and at least one cross-linkable moiety to the terminalgroups of each dendritic branch, and where the cross-linkable moiety isbonded to the terminal group via the labile bond; cross-linking thecross-linkable moieties of adjacent dendritic branches; and cleaving thelabile bonds, thereby freeing the dendrimer and forming a moleculeencapsulated within a cross-linked shell molecule.

The present invention further provides a process for producing asupramolecular structure comprising: reacting a multi-generationdendrimer with a monomer, where the dendrimer comprises a core molecule,a plurality of interior generations spherically disposed around the coremolecule and an outermost generation comprising a plurality of dendriticbranches having terminal groups sufficiently reactive to undergoaddition or substitution reactions, where the monomer introduces alabile bond and at least one cross-linkable moiety to the terminalreactive groups, and where the cross-linkable moiety is bonded to theterminal reactive group via the labile bond; cross-linking thecross-linkable moieties of adjacent dendritic branches; and cleaving thelabile bonds, thereby freeing the dendrimer and forming a moleculeencapsulated within a cross-linkable shell molecule.

The present invention further provides a supramolecular structureproduced by the process comprising: reacting a multi-generationdendrimer with a monomer, where the dendrimer comprises a core molecule,a plurality of interior generations spherically disposed around the coremolecule and an outermost generation comprising a plurality of dendriticbranches having terminal groups sufficiently reactive to undergoaddition or substitution reactions, where the monomer introduces alabile bond and at least one cross-linkable moiety to the terminalgroups, and where the cross-linkable moiety is bonded to the terminalgroup via the labile bond; crosslinking the cross-linkable moieties ofadjacent dendritic branches; cleaving the labile bonds, thereby freeingthe dendrimer and forming a molecule encapsulated within a cross-linkedshell molecule; and degrading and removing the free dendrimer, therebyproducing a intramolecularly cross-linked spherical hollow shellstructure.

A process for producing a supramolecular structure comprising: reactinga multi-generation dendrimer with a monomer, where the dendrimercomprises a core molecule, a plurality of interior generationsspherically disposed around the core molecule and an outermostgeneration comprising a plurality of dendritic branches having terminalgroups sufficiently reactive to undergo addition or substitutionreactions, where the monomer introduces at least one labile bond and across-linkable moiety to the terminal group of each dendritic branch,and where the cross-linkable moiety is bonded to the terminal group viathe labile bond; cross-linking the cross-linkable moieties of adjacentdendritic branches; cleaving the labile bonds, thereby freeing the coredendrimer and forming a molecule encapsulated within a cross-linkedshell molecule; and degrading and removing the free dendrimer, therebyproducing a intramolecularly cross-linked spherical hollow shellstructure.

The present invention also provides a supramolecular structurecomprising a dendrimer having a cross-linked peripheral surface; asupramolecular structure comprising a hollow cross-linked shellmolecule; and a supramolecular structure comprising a dendrimer and across-linked shell molecule spherically disposed about the dendrimer,wherein the dendrimer and the cross-linked shell molecule are notionically or covalently bonded together.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the reaction of a dendrimer anda monomer to produce a dendrimer having a cross-linkable moiety on theterminal end of each dendritic branch of the outermost generation of thedendrimer.

FIG. 2 is a schematic representation of a poly(propyleneimine) dendrimerhaving a cross-linked peripheral surface pursuant to the presentinvention.

FIG. 3 is a schematic representation of a dendrimer having twocross-linkable moieties on the terminal end of each dendritic branch ofthe outermost generation.

DETAILED DESCRIPTION OF THE INVENTION

It has now been found that dendritic structures that contain or aremodified to contain terminal or peripheral cross-linkable moieties canbe cross-linked to form supramolecular structures. Once cross-linked,the resultant interconnected peripheral surface network may be cleavedor severed from the core molecule to form an independent shell networkthat is not chemically bonded to the core molecule. In anotherembodiment of the present invention, the core molecule, which caninclude a majority of the dendritic network, can be degraded andremoved. Where the core is left intact and only severed from the shell,a molecule encapsulated within a cross-linked shell molecule is formed.Where the core molecule is degraded, a porous, hollow spherical shellstructure simply remains. The supramolecular structures of the presentinvention are porous and are capable of entrapping or encapsulatingsmaller molecules, and are capable of allowing small molecules or thelike to pass into or out of its network.

Because the supramolecular shells of the present invention are formed bycross-linking the surface of dendritic molecules, the surface of theresultant molecular shell will be porous. The porosity of the shell is afunction of many variables, including, but not limited to, the size ofthe dendrimer that is cross-linked, the dendrimer's density at thesurface of the dendrimer, the number of cross-linkable moieties orreactive sites within the periphery monomers of the dendrimer, and thecrosslinking agents employed.

In one embodiment, the present invention provides a supramolecularstructure produced by cross-linking the periphery of core dendrimer andsevering the core from the cross-linked network. The core dendrimercomprises a core molecule, a plurality of interior generations, eachgeneration comprising a plurality of dendritic branches sphericallydisposed around the core molecule and an outermost generation orperiphery comprising a plurality of dendritic branches having terminalreactive groups sufficiently reactive to undergo addition orsubstitution reactions. The core dendrimer is reacted with a monomerthereby resulting in the addition of a generation to the dendrimer. Themonomer also introduces a labile bond and at least one cross-linkablemoiety to the terminal reactive group of each dendritic branch of theoutermost generation or periphery. The cross-linkable moiety is bondedto the terminal groups via the labile bond. The cross-linkable moietiesof adjacent dendritic branches are then cross-linked with across-linking agent to produce a dendrimer having a cross-linkedperipheral surface. The labile bonds of each dentritic branch aresubsequently cleaved, thereby freeing the core dendrimer and forming afreed molecule encapsulated within a cross-linked shell molecule. Thefreed dendrimer and cross-linked shell structure are not ionically orcovalently bonded together.

A dendrimer or dendritic structure is a unimolecular assemblage havingthree distinguishing architectural features, namely, a core; interiorgeneration layers or interior generations; and exterior surface ofterminal moieties attached to the outermost generation. The topology ofa dendrimer is achieved by the ordered assembly of repeating units inconcentric, dendritic tiers around a core molecule. The size and shapeof dendrimers and the moieties present in the dendrimer molecule can becontrolled by the choice of the core, the number of generations employedin creating the dendrimer, and the choice of the repeating units at eachgeneration.

Any dendrimer molecule can be utilized in the practice of the presentinvention. Preferred dendrimers include nitrogen-containing andcarbosilane dendrimers, such as poly(propyleneimine) (DAB) andpolyamidoamine (PAMAM) dendrimers. It is also preferred that thedendrimers have sufficient density at their periphery, although itshould be appreciated that any dendrimer can be modified to yield anadequate peripheral density. Inasmuch as there are a number ofdendrimers that can or have been synthesized, the skilled artisanpracticing the present invention can simply select a dendrimer withdendrictic branches having a cross-linkable end or terminal groups, orthe cross-linkable groups can be added to the periphery of thedendrimer. Other useful dendrimer molecules include, for example andwithout limitation, those described in U.S. Pat. Nos. 4,507,466,4,558,120, 4,568,737 and 4,587,329 as well as those described inDendritic Molecules, Concepts, Syntheses, Perspectives. Newkome et al.,VCH Publishers, Inc. New York, N.Y. (1996).

As mentioned above, the core could contain a variety of functionalitiesthat would prove useful in the final device. Commercially availablepoly(propylenimine) DAB series and poly(amidoamine) (PAMAM) dendrimersare useful as the core dendrimer. These dendrimers are available in anumber of generations. For example, DAB-16 is the parent thirdgeneration dendrimer with 16 terminal NH₂ groups, DAB-32 is a fourthgeneration dendrimer terminated with 32 NH₂ groups, and DAB-64 is theanalogous fifth generation dendrimer with 64 terminal NH₂ groups.

The periphery moieties of the precursor dendrimers must becross-linkable or polymerizable. In other words, the ultimate orexternal generation must include or be formed from monomers or moietiesthat have reactive moieties that are sufficiently reactive to undergoaddition or substitution reactions at the terminal portions of eachdendritic branch of the ultimate generation. It is preferred that theseterminal monomers include at least three reactive moieties, especiallyin situations where the outer shell will ultimately be severed or freedfrom the inner core molecule. Preferred periphery moieties include thosethat contain some level of unsaturation.

Typically, commercially available dendrimers do not contain terminalgroups or moieties that are easily cross-linkable. Therefore, it isdesirable to modify the base dendrimer by introducing cross-linkablemoieties at the periphery of the core dendrimer. The skilled artisanwill be able to readily determine the proper reaction conditionsnecessary to add cross-linkable monomeric units to the terminal portionof each dendritic branch of the core or precursor dendritic molecule.

Suitable monomers, such as trialkenesilanes, can be used to introduceunsaturated cross-linkable moieties to the terminal reactive groups ofeach dendritic branch. These reactive trialkenesilane monomers include,but are not limited to, trivinylchlorosilane, triallychlorosilane,tripropargylchlorosilane and diallylchlorosilane. Trivinylchlorosilaneis the most preferred reactive monomer. Depending on the polymerizationor cross-linking technique employed, a number of polymerizable endgroups can be employed. The resulting structure is a dendrimer havingcross-linkable moieties bonded to the terminal end of the dendriticbranches comprising the outermost or peripheral generation of thedendrimer. The reaction of a poly(propylenimine) (DAB) dendrimer, whichhas dendritic branches with terminal NH₂ groups, withtrivinylchlorosilane results in the quantitative conversion of terminalNH₂ groups to NH-Si(CH=CH₂)₃ groups.

In this embodiment, it is preferred that the reactive monomer introducea labile bond to the terminal reactive group of each dendritic branch.Useful labile bonds include, but are not limited to silicon-oxygen,oxygen-nitrogen, silicon-oxygen-carbon, nitrogen-silicon,nitrogen-carbonyl-nitrogen, silicon-acetylene, amide, blockedisocyanates and urea bonds. The most preferred labile bond is asilicon-nitrogen bond.

As mentioned above, the cross-linkable moitites bonded to the terminalend of the dendritic branches comprising the outermost or peripheralgeneration of the dendrimer are cross-linked. Suitable crosslinkingtechniques include hydrosilylation, olefin metathesis reactions, radicalpolymerizations, controlled radical polymerization, polycondensationreactions, anionic and cationic polymerizations, and coordinationpolymerization reactions including Ziegler-Natta polymerizationreactions. The preferred method for crosslinking the cross-linkablemoieties is hydrosilylation, which can be employed to cross-linkunsaturated organic groups.

Where hydrosilylation chemistry is employed as the crosslinking method,it is preferred to crosslink the cross-linkable moieties with a monomeror crosslinking agent. Suitable crosslinking agents are selected fromdouble and multiple crosslinking agents. The double crosslinking agentsrefer to those agents that have two Si—H functional groups that canhydrosilate the terminal unsaturated cross-linkable moieties, which arepreferably alkene groups. Multiple cross-linking agents are those agentsthat have more than two Si—H functional groups that can hydrosilatealkene groups. The preferred double cross-linking agents have thegeneral formula I:

where R₁ is selected from hydrogen or organic groups having from about 1to about 30 carbon atoms, R₂ is selected from hydrogen and organicgroups having from about 1 to about 30 carbon atoms, and x is an integerfrom about 1 to about 4. Preferably, R₁ is selected from hydrogen ororganic groups having from about 1 to about 15 carbon atoms, R₂ isselected from hydrogen and organic groups having from about 1 to about15 carbon atoms, and x is an integer from about 1 to about 2. Even morepreferably, R₁ and R₂ have less than about 5 carbon atoms.

Nonlimiting examples of double cross-linking agents defined by formula Iinclude H(CH₃)₂Si(CH₂CH₂)SiH(CH₃)₂, and H(C₃H₇)Si(CH₂CH₂)SiH(C₃H₇).Oxygen based curatives may also be employed such astetramethyldisiloxane (H(CH₃)₂Si(O)SiH(CH₃)₂).

Suitable multiple cross-linking agents for crosslinking for the terminalcross-linkable moieties of the supramolecular structures of the presentinvention include, but are not limited to, CH₃Si(CH₂CH₂Si(CH₃)₂H)₃,CH₃(CH₂SiH₂)₂CH₃, HC(Si(R¹)₂H)₃, Si(R¹)₂H₂, and (SiR¹H—O—)₄; wherein R¹is selected from the group consisting of hydrogen and organic groupshaving from about 1 to about 15 carbon atoms.

The multiple cross-linking agents may also include linear polymershaving Si-H functionalities as part of or pendant from the polymerchain, cyclic compounds having Si-H functionalities as part of orpendant from the ring, a dendrimer having Si-H functionalities, andmixtures of these three types of cross-linking agents. Useful linearpolymers include, but are not limited to,(CH₃)₃Si—O—(SiR²H—O)_(n)—Si(CH₃)₃, wherein R is selected from methyl andethyl groups and wherein n is a positive integer from about 10 to about100, preferably about 10 to about 50 and most preferably from about 10to about 30; H(CH₃)₂Si—O—(SiPh(—OSi(CH₃)₂H)—O)_(n)—Si(CH₃)₂H, wherein nis a positive integer from about 10 to about 100, preferably about 10 toabout 50 and most preferably from about 10 to about 30;(CH₃)₃Si—O—(Si(CH₃)(H)—O)_(m)—(Si(CH₃)(C₈H₁₇)—O)_(n)—Si(CH₃)₃, wherein mis a positive integer from about 10 to about 100, preferably about 10 toabout 50 and most preferably from about 10 to about 30, and wherein n isa positive integer from about 10 to about 100, preferably about 10 toabout 50 and most preferably from about 10 to about 30; andH₂R³Si(SiR³H)_(n)—SiR³H₂; wherein R is selected from alkyl and arylgroups having from about 1 to about 15 carbon atoms, and wherein n is apositive integer from about 10 to about 100, preferably about 10 toabout 50 and most preferably from about 10 to about 30. The selection ofa suitable cross-linking agent should not limit the scope of theinvention.

It should be noted that the Si—H functionalities of the double andmultiple cross-linking agent may be replaced by any functional groupthat adds across a double bond, and that capable of cross-linking alkenegroups on the peripheral generation or surface of the dendrimer.Suitable functional groups that may replace the Si—H cross-linkinggroups include, but are not limited to B—H, P—H, As—H, and M—H, whereinM is selected from d-block and p-block metals. Suitable p-block metalsinclude, but are not limited to Ge and Sn.

Without being bound to any particular theory, the linear polymer maycrosslink the dendrimer surface by wrapping itself around the outerperiphery of the dendrimer, and cross-linking the cross-linkablemoieties of the peripheral surface of the dendrimer. The linear polymermust be long enough to wrap around a significant portion of thedendrimer and, therefore, the length of the dendrimer can vary basedupon the size of the dendrimer to be cross-linked. Furthermore, severallinear polymer chains may be required to fully cross-link the dendrimerand there may be overlapping of the linear polymer cross-linking agentson the dendrimer surface.

Hydrosilylation (also known as hydrosilation) reactions are carried outunder dilute conditions in order to favor intramolecular crosslinkingover intramolecular cross-linking, and typically take place in thepresence of a catalyst such as the Karstedt's catalyst, which is aplatinum alkene. Karstedt's catalyst is commercially available fromGelest. Other catalysts that can be employed include H₂PPCl₆ andCr(CO)₆.

Before cross-linking of the core dendrimer, the labile bonds aresusceptible to hydrolysis with water. In contrast, a dendrimer having across-linked peripheral network is stable in water. The dendrimer havinga cross-linked peripheral network requires dilute acid, such ashydrochloric acid, instead of water to liberate free dendrimer and thecross-linked shell structure as separate molecules.

As those skilled in the art will appreciate, of the crosslinking agent,which is a function of the integer x, will affect the rigidity and poresize of the interconnected surface network. Shorter organic chains willprovide a more rigid shell with less permeability, while longer chainswill provide a more flexible shell with greater permeability.

With reference to formula I, it should also be appreciated that R₁ andR₂ can be selected to provide further functionality to the molecule. Forexample, R₁ and R₂ can include moieties, such as sulfates, carboxylate,or other hydrophilic moieties, that can provide greater solubility tothe molecule in hydrophilic systems, or R₁ and R₂ can include a longchain alkyl or similar moiety, that can provide greater solubility tothe molecule in lipophilic environments.

As mentioned hereinabove, the term organic group, as used herein, caninclude any carbon based group, moiety, or radical having from about 1to about 15 carbon atoms. Preferably, the organic group is a group,moiety or radical that will not inhibit the hydrosilation reaction, orotherwise deleteriously impact the supramolecular structures of thepresent invention. Additionally, the carbon based groups can includehetero atoms such as oxygen, nitrogen, sulfur, phosphorous, or silicon.Or, for purposes of this specification, the term organic groups willlikewise refer to silicon based groups such as silicon chloride groups.Specifically, the carbon based groups can be aliphatic, cycloaliphatic,and aromatic groups. The aliphatic groups can be saturated orunsaturated and therefore can include alkanes, alkenes, alkynes, alkoxysand polyethers. Exemplary organic groups include alkyl groups, carboxylgroups, alcohol groups, and amino groups.

Alternatively, intramolecular cross-linking of a dendrimer havingunsaturated cross-linkable moieties can be achieved by olefin metathesisby using ring opening metathesis polymerization (ROMP) or acyclic dienemetathesis (ADMET) catalysts. Because no cross-linking agent, per se, isemployed that will add to the intramolecular cross-linked network, therigidity and permeability of the molecular shells produced by olefinmetathesis is controlled by the size and nature of the substituents ofthe dendrimer. Diallylsilanes are quite active in olefin metathesis andare reported to give high yield polymers at low temperature. Subjectingthe triallylsilane covered dendrimers to olefin metathesis may initiallygive the formation of five membered rings when an allyl group on asilicon undergoes olefin metathesis with another allyl group on the samesilicon; however, cyclopentenes undergo productive ring openingmetathesis polymerizations (ROMP).

A variety of catalysts can be employed to accomplish olefin metathesis.These catalysts include, but are not limited to, Osborne's, Schrock's,and Grubb's catalysts. As is generally known in the art, Schrock'scatalyst is a molybdenum-tungsten-carbene complex, and Grubb's catalystis a ruthenium-carbene-phosphine complex.

For example, a dendrimer having an outer periphery of dialkyl silanegroups will give rise to the formation of a five membered ring when anallyl group on a silicon atom undergoes olefin metathesis with anotherallyl group on the same silicon atom.

It should be understood that the nature of the crosslinkedinterconnected surface is dependent on the periphery groups. Forexample, vinyl silane covered or terminated dendrimers do not undergohomopolymerzation, but rather undergo copolymerization with added dienessuch as 1,9-decadiene. Divinyl dimethylsilane, however, undergoeshomopolymerization with RuCl₂(Ph₃)₃ to give silyene-vinylene oligomersby acyclic diene polymerization.

As mentioned above, once the outermost generation of the core dendrimerhas been cross-linked, the labile bonds connecting the cross-linkedmoieties to the core dendrimer are cleaved or severed, thereby freeingthe core dendrimer from the cross-linked shell molecule. Depending onthe labile bond that exists in each dendritic branch, a number oftechniques may be employed to sever or cleave the labile bonds. Forexample, one method that can be employed when the labile bond is anitrogen-silicon bond includes placing the cross-linked core dendrimerstructure in a dilute acidic solution, such as hydrochloric acid.

Those skilled in the art will appreciate that the core molecule, i.e.,that molecule that has been entrapped within the super-molecularstructure of the present invention, can be the starting or precursordendrimer, or a derivative thereof. This is especially true where thelabile bond is positioned farther from the core of the precursordendrimer. It is also possible, however, to select or place the labilebond closer to or adjacent to the core of the precursor dendrimer. Insome situations, the freed cored may be small enough to escape out ofthe interconnected shell network. It should also be understood thatwhere the labile bond is not adjacent to the outer cross-linkedperiphery, moieties that were once a part of the precursor dendriticnetwork will extend from the cross-linked surface. These moieties willtypically extend inward into the shell, but, based on the environmentand porosity of the surface, these moieties may extend outwardly aswell.

In another embodiment, the present invention provides a supramolecularstructure produced by crosslinking the periphery of a core dendrimer,severing the core from the cross-linked network and degrading the innercore molecule. The core dendrimer comprises a core molecule, a pluralityof interior generations, each generation comprising a plurality ofdendritic branches spherically disposed around the core molecule, and anoutermost or periphery generation comprising a plurality of dendriticbranches having terminal groups sufficiently reactive to undergoaddition or substitution reactions. The core dendrimer is reacted withamonomer thereby resulting in the addition of a generation to thedendrimer. The monomer also introduces a labile bond and at least onecross-linkable moiety to the terminal group of each dendritic branch ofthe outermost generation or periphery. The cross-linkable moiety isbonded to the terminal group via the labile bond. The cross-linkablemoieties of adjacent dendritic branches are then cross-linked asdisclosed hereinabove. The labile bonds of each dentritic branch arecleaved, thereby freeing the core dendrimer and forming a moleculeencapsulated within a cross-linkable shell structure. The free dendrimeris subsequently degraded and removed, thereby producing aintramolecularly cross-linked shell structure that is empty or hollow.The dendrimer is prepared in the same manner as described hereinaboveand the core or precursor dendrimer is chosen from dendrimers having abranched network comprising many labile bonds.

The severing of the labile bonds within the branched network of the coreor precursor dendrimer can cause the complete destruction of the innerdendritic network. Depending on the size of the resulting fragmentedpieces of the precursor dendrimer, the outer cross-linked surface mayallow the fragmented pieces to pass through the wall of the shell andinto the surrounding medium. In the event that all of the fragmentedpieces are able to escape from the shell structure or are otherwiseremoved, the supramolecular structure of the present invention willsimply be an empty or hollow cross-linked shell molecule.

The synthetic route to producing the supramolecular structures of thepresent invention may include the incorporation of at least one labilegeneration within the dendritic structure of the base or precursordendrimer. This is preferred where it is desired to sever the outerinterconnected shell network form the inner core of the molecule andthereby form a shell molecule having a spherical topology. As theskilled artisan will appreciate, the labile bond will be incorporatedinto the dendritic network simultaneously with the addition of theperipheral generation that will ultimately be cross-linked to form theinterconnected shell network. It should also be appreciated that themonomers used to form the periphery generation may themselves contain alabile bond that can be employed to sever the shell from the core. Stillfurther, the starting or precursor dendritic structure may contain oneor more labile bonds. In the latter two situations, therefore, theincorporation of a labile bond into the dendritic network is not anecessary step toward achieving the molecules of the present invention.

The skilled artisan will readily understand what is meant by the termlabile bond. Preferably, these bonds are those chemical bonds betweenatoms within a molecule that can be severed under controlled conditionsor when desired. For example, it is known that silicon-acetylene bondscan be severed in the presence of potassium fluoride. Other labile bondsinclude, for example and without limitation, silicon-oxygen bonds,silicon-nitrogen bonds, oxygen-nitrogen bonds,nitrogen-carbonyl-nitrogen bonds, blocked isocyanates and ureas.

In another embodiment, the present invention provides a supramolecularstructure produced by cross-linking the peripheral or outermostgeneration of a core dendrimer. The core dendrimer comprises a coremolecule, a plurality of interior generations each interior generationcomprising a plurality of dendritic branches, that are sphericallydisposed around the core molecule, and an outermost or peripheralgeneration comprising a plurality of dendritic branches having terminalreactive groups sufficiently reactive to undergo addition orsubstitution reactions. The core dendrimer is reacted with a monomerthereby resulting in the addition of a generation to the dendrimer. Themonomer also introduces at least one cross-linkable moiety to theterminal reactive group of each dendritic branch of the outermost orperipheral generation. In this embodiment, the cross-linkable moiety mayor may not be bonded to the terminal reactive groups via a labile bond.The cross-linkable moieties of adjacent dendritic branches are thencross-linked with a cross-linking agent to produce a dendrimer having across-linked peripheral surface.

The supramolecular structures of the present invention have severalpotential applications including, but not limited to, applications suchas catalytic sites for sequential catalytic reactions and as shape andsize selective catalysts, ferromagnetic sites for memory and otherelectronic devices, sensor sites for sensing chemical molecules, andmolecular containers, which might slowly release reactive reagents forperforming reactions under high dilution conditions.

For example, in the supramolecular shell structures of the presentinvention, the hollow shell may be porous and allow the flow of smallmolecules to the central core. The central molecule may containcatalytic centers, electrochemical centers, or other types of centersthat must be isolated from other centers on the surface of the shellmolecule or from the flow medium outside the supramolecular shellstructure. The supramolecular shell structure could serve as a housingunit for moieties that can catalyze two sequential reactions where twocatalytic moieties would not function if in close physical contact, amoiety that can conduct catalytic chemistry at the core but can beisolated from the flowstream of the plant and might then be removed fromthe flowstream of the plant by filtration, and a sensor with an exteriorshell permeable to some chemicals but not to others, and with a corethat could give a color change or other readout when interacting withthe chemical to be sensed. Several concentric shells could besynthesized to give the utmost selectivity and sequestering ability.

To assist with the understanding of this invention is a glossary ofterms and definitions that are used throughout the specification areprovided below.

The terms “supramolecular structure” and “supramolecular assembly,” asused throughout this specification, may be used interchangeably andrefer to a molecular assembly in which one molecular structure isenclosed within the cross-linked shell of another molecular structure.The encapsulated structure and the shell structure may or may not becovalently or ionically bonded together; therefore, the supramolecularstructure can be simplistically thought of as a core molecular structureretained within a cross-linked shell structure. The terms“supramolecular structure” and “supramolecular assembly” also refer toporous, hollow cross-linked shell structures.

The term “cross-linkable moieties” refer to moieties that aresufficiently reactive to undergo addition or substitution reactions withadjacent cross-linkable moieties to form a shell structure.

The term “core dendrimer” refers to the precursor structure of thesupramolecular structure. The core dendrimer of the supramolecularstructure of the present invention can be freed upon the destruction ofthe labile bonds connecting the core to the cross-linkable moities.

The term “generation” refers to each successive concentric layer addedto the core molecule in the iterative formation of a dendriticstructure. The first generation is the monomer layer initially bound tothe core molecule while successive generations, for example, the second,third and fourth generations, are bound to the preceding generation. Thefirst generation begins the growth of the dendritic branches.

The term “dendritic branches” refers to each monomer chain extendingfrom the core of the dendrimer or some common atom and therefore eachsplit or branch in a chain can be considered a dendritic branch.

The term “peripheral generation” of the dendrimer or dendritic structurerefers to the outermost generation or the generation furthest from thecore molecule. The peripheral generation provides a plurality ofdendritic branches to which the labile bonds and cross-linkable moietiesmay be attached.

The term “sufficiently dense” refers to a supramolecular structurehaving an adequate number of cross-linkable moieties on the periphery ofthe structure to effect cross-linking.

The term “double cross-linking agent” refers to a cross-linking agent orcross-linker that has two Si—H functional groups that can hydrosilatealkene groups. The term “multiple cross-linking agent” refers to across-linking agent or cross-linker that has more than two Si—Hfunctional groups that can hydrosilate alkene groups.

In order to demonstrate the practice of the present invention, thefollowing examples have been prepared and tested as described in theGeneral Experimentation Section disclosed hereinbelow. The examplesshould not, however, be viewed as limiting the scope of the invention.The claims will serve to define the invention.

GENERAL EXPERIMENTAL

Example Nos. 1-5, herein below describe the process for producing atrivinylchlorosilane covered polypropyleimine (DAB) dendrimer. Thereactions were carried out in an inert atmosphere at room temperatureand atmospheric pressure. The dendrimer was initially dissolved intoluene or tetrahydrofuran (THF) and the triethylamine andtrivinylchlorosilane were subsequently added, respectively. The productwas evidenced by the presence of a pale yellow oily product. NMRanalysis indicated that the desired product, namely atrivinylchlorosilane-covered dendrimer, had been achieved.

EXAMPLE 1

1.039 grams of DAB-4 dendrimer, a 1.0 generation polypropyleiminedendrimer, was initially dissolved in 30 milliliters of tetrahydrofuran(THF) and was reacted with 3.05 milliliters of trivinylchlorosilane inthe presence of 2.74 milliliters of triethylamine. The reaction mixturewas stirred overnight.

EXAMPLE 2

2 grams of DAB-8 dendrimer, a 2.0 generation polypropyleimine dendrimer,was initially dissolved in 40 milliliters of toluene, and reacted with4.01 milliliters of trivinylchlorosilane in the presence of 3.61milliliters of triethylamine. The reaction mixture was stirredovernight.

EXAMPLE 3

0.5 grams of DAB-16, a 3.0 generation polypropyleimine dendrimer, wasinitially dissolved in 30 milliliters of toluene, and reacted with 1.10milliliters of trivinylchlorosilane in the presence of 0.99 millilitersof triethylamine. The reaction mixture was stirred overnight.

EXAMPLE 4

1.423 grams of DAB-32, a 4.0 generation polypropyeimine dendrimer, wasinitially dissolved in 40 milliliters of toluene, and reacted with 3.10milliliters of trivinylchlorosilane in the presence of 2.7 millilitersof triethylamine. The reaction mixture was stirred overnight.

EXAMPLE 5

0.8 grams of DAB-64, a 5.0 generation polypropyleimine dendrimer, wasinitially dissolved in 30 milliliters of toluene and reacted with 1.24milliliters of trivinylchlorosilane in the presence of 1.38 millilitersof triethylamine. After stirring the reaction overnight, the reactionmixture was filtered and the volatiles were removed at reduced pressure.

The results of NMR analysis on Example Nos. 1-5, above, indicate thatthe reaction of the terminal primary amine groups of the dendriticbranches of the peripheral generation with chlorosilanes results in theformation of trinvinylchlorosilane covered polypropyleimine (DAB)dendrimers, with a high yield conversion of terminal amine groups toNH—Si(CH═CH₂)₃ groups.

Example Nos. 6-10, below, demonstrate the crosslinking oftrinvinylchlorosilane covered polypropyleimine (DAB) dendrimers byhydrosilation with different crosslinking agents.

EXAMPLE 6

Using the trivinylchlorosilane covered polypropyleimine (DAB-16)dendrimer prepared in Example 3, hereinabove, peripheral trivinylsilylgroups of adjacent dendritic branches were cross-linked usingtetramethyldisilethylene as the crosslinking agent. Specifically, about0.30 grams of the trinvinylchlorosilane covered polypropyleimine (DAB)dendrimer prepared in Example 3 (modified DAB-16 dendrimer), wasdissolved in about 5 ml of tetrahydrofuran (THF). The dissolveddendrimer was reacted with about 0.39 milliliters oftetramethyldisilethylene crosslinking agent in the presence of a fewdrops of Karstedt's catalyst. The reaction mixture was stirred andheated at 55° C. for 3 hours. 10 milliliters of tetrahydrofuran wasadded to the mixture and the reaction was stirred for another 4 hours at55° C.

EXAMPLE 7

Using the trinvinylchlorosilane covered polypropyleimine (DAB) dendrimerprepared in Example 3, hereinabove, peripheral trivinylsilyl groups ofadjacent dendritic branches were cross-linked usingtetramethyldisilethylene as the crosslinking agent. Specifically, about0.126 grams of the trinvinylchlorosilane covered polypropyleimine (DAB)dendrimer prepared in Example 3 (modified DAB-16 dendrimer), wasdissolved in about 60 ml of tetrahydrofuran (THF). The dissolveddendrimer was reacted with about 0.165 milliliters oftetramethyldisilethylene crosslinking agent in the presence of a fewdrops of Karstedt's catalyst. The reaction mixture was stirred andheated at 60° C. for 6 hours.

EXAMPLE 8

Using the trinvinylchlorosilane covered polypropyleimine (DAB) dendrimerprepared in Example 3, hereinabove, peripheral trivinylsilyl groups ofadjacent dendritic branches were cross-linked usingtetramethyldisiloxane as the crosslinking agent. Specifically, about 0.3grams of the trinvinylchlorosilane covered polypropyleimine (DAB)dendrimer prepared in Example 3 (modified DAB-16 dendrimer), wasdissolved in about 60 ml of tetrahydrofuran (THF). The dissolveddendrimer was reacted with about 0.75 milliliters oftetramethyldisiloxane crosslinking agent in the presence of a few dropsof Karstedt's catalyst. The reaction mixture was stirred and heated at60° C. for 1 day.

EXAMPLE 9

Using the trinvinylchlorosilane covered polypropyleimine (DAB) dendrimerprepared in Example 3, hereinabove, peripheral trivinylsilyl groups ofadjacent dendritic branches were cross-linked using hexysilane as thecrosslinking agent. Specifically, about 0.3 grams of thetrinvinylchlorosilane covered polypropyleimine (DAB) dendrimer preparedin Example 3 (modified DAB-16 dendrimer), was dissolved in about 200 mlof tetrahydrofuran (THF). The dissolved dendrimer was reacted with about0.204 milliliters of hexysilane crosslinking agent in the presence of afew drops of Karstedt's catalyst. The reaction mixture was stirred andrefluxed for 2 days.

EXAMPLE 10

Using the trinvinylchlorosilane covered polypropyleimine (DAB) dendrimerprepared in Example 3, hereinabove, peripheral trivinylsilyl groups ofadjacent dendritic branches were cross-linked usingtetramethylcyclotetrasiloxane as the crosslinking agent. Specifically,about 0.3 grams of the trinvinylchlorosilane covered polypropyleimine(DAB) dendrimer prepared in Example 3 (modified DAB-16 dendrimer), wasdissolved in about 200 ml of tetrahydrofuran (THF). The dissolveddendrimer was reacted with about 0.338 milliliters oftetramethylcyclotetrasiloxane crosslinking agent in the presence of afew drops of Karstedt's catalyst. The reaction mixture was stirred andrefluxed for 2 days.

The results of NMR analysis on Example Nos. 6-10, above, indicate thecompletion of the hydrosilation reactions of the terminal crosslinakblemoieties of the dendritic branches of the peripheral generation to forma cross-linked shell structure.

Example Nos. 11-13, below, demonstrate the crosslinking oftrinvinylchlorosilane covered fourth generation polypropyleimine(DAB-32) dendrimers by hydrosilation with different crosslinking agents.

EXAMPLE 11

Using the trinvinylchlorosilane covered DAB-32 dendrimer prepared inExample 4, hereinabove, the peripheral trivinylsilyl moieties werecross-linked using tetramethyldisilethylene. Specifically, about 0.52grams of the modified DAB-32 dendrimer was initially dissolved in 60milliliters of tetrahydrofuran, and reacted with about 0.67 grams of thetetramethyldisilethylene crosslinking agent in the presence of aboutfive drops of Karstedt's catalyst. The reaction mixture was stirred andheated at 60° C. for 6 hours. An additional 0.33 milliliters oftetramethyldisilethylene was added and the reaction mixture was stirredand heated for 3 hours at 60° C.

EXAMPLE 12

Using the trinvinylchlorosilane covered DAB-32 dendrimer prepared inExample 4, hereinabove, the peripheral trivinylsilyl moieties werecross-linked using MeSi(CH₂CH₂SiMe₂H)₃. Specifically, about 0.2 grams ofthe modified DAB-32 dendrimer was initially dissolved in 60 millilitersof tetrahydrofuran, and reacted with about 0.279 grams of theMeSi(CH₂CH₂SiMe₂H)₃ crosslinking agent in the presence of about fivedrops of Karstedt's catalyst and 10 milliliters of tetrahydrofuran. Thereaction mixture was stirred and refluxed for 2 days.

EXAMPLE 13

Using the trinvinylchlorosilane covered DAB-32 dendrimer prepared inExample 4, hereinabove, the peripheral trivinylsilyl moieties werecross-linked using tetramethyldisiloxane. Specifically, about 0.2 gramsof the modified DAB-32 dendrimer was initially dissolved in 60milliliters of tetrahydrofuran, and reacted with about 0.4 grams of thetetramethyldisiloxane crosslinking agent in the presence of about fivedrops of Karstedt's catalyst and 20 milliliters of tetrahydrofuran. Thereaction mixture was stirred and heated at 60° C. for 1 day.

The volatiles were removed from Example Nos. 11-13 at reduced pressureto provide a cross-linked compound, which was a yellow, oily solid. Theresults of NMR analysis on Example Nos. 11-13, above, indicate thecompletion of the hydrosilation reactions of the terminal cross-linkablemoieties of the dendritic branches of the peripheral generation to forma cross-linked shell structure.

Example Nos. 14-17, below, demonstrate the crosslinking oftrinvinylchlorosilane covered fifth generation polypropyleimine (DAB-64)dendrimers by hydrosilation with different crosslinking agents.

EXAMPLE 14

Using the trinvinylchlorosilane covered DAB-64 dendrimer prepared inExample 5, hereinabove, the peripheral trivinylsilyl moieties werecross-linked using tetramethyldisiloxane. Specifically, about 0.2 gramsof the modified DAB-64 dendrimer was initially dissolved in 60milliliters of tetrahydrofuran, and reacted with about 0.220 grams ofthe tetramethyldisiloxane crosslinking agent in the presence of aboutfive drops of Karstedt's catalyst and 50 milliliters of tetrahydrofuran.The reaction mixture was stirred and refluxed for 1 day.

EXAMPLE 15

Using the trivinylchlorosilane covered DAB-64 dendrimer prepared inExample 5, hereinabove, the peripheral trivinylsilyl moieties werecross-linked using 1,1-dichloro-,3,5-dimethyl-1,3-disilabutane.Specifically, about 0.177 grams of the modified DAB-64 dendrimer wasinitially dissolved in 70 milliliters of tetrahydrofuran, and reactedwith about 0.868 grams of the1,1-dichloro-,3,5-dimethyl-1,3-disilabutane crosslinking agent in thepresence of about five drops of Karstedt's catalyst and 0.22 grams oftetramethyldisiloxane. The reaction mixture was stirred and refluxed for1 day.

EXAMPLE 16

Using the trinvinylchlorosilane covered DAB-64 dendrimer prepared inExample 5, hereinabove, the peripheral trivinylsilyl moieties werecrosslinked using MeSi(CH₂CH₂SiMe₂H)₃. Specifically, about 0.1 grams ofthe modified DAB-64 dendrimer was initially dissolved in 60 millilitersof tetrahydrofuran, and reacted with about 0.184 grams of theMeSi(CH₂CH₂SiMe₂H)₃ crosslinking agent in the presence of about fivedrops of Karstedt's catalyst and 10 milliliters of tetrahydrofuran. Thereaction mixture was stirred and refluxed for 2 days.

EXAMPLE 17

Using the trinvinylchlorosilane covered DAB-64 dendrimer prepared inExample 5, hereinabove, the peripheral trivinylsilyl moieties werecross-linked using octakis(dimethylsiloxy)Ts-silsequioxane.Specifically, about 0.15 grams of the modified DAB-64 dendrimer wasinitially dissolved in 120 milliliters of Et₂O. A solution containing 5drops of Karstedt's catalyst and 0.457 grams ofoctakis(dimethylsiloxy)T₈-silsequioxane crosslinking agent in 40milliliters of tetrahydrofuran was added dropwise to the modified DAB-64dendrimer. The reaction mixture was stirred and refluxed for 3 days.

The volatiles were removed from Example Nos. 14-17 at reduced pressureto provide a cross-linked compound, which was a yellow, oily solid. Theresults of NMR analysis on Example Nos. 14-17, above, indicate thecompletion of the hydrosilation reactions of the terminal cross-linkablemoieties of the dendritic branches of the peripheral generation to forma cross-linked shell structure.

The labile bonds of the dendritic branches were cleaved with aqueoushydrochloric acid, thereby freeing the core dendrimer from thecross-linked peripheral generation to form a molecule encapsulatedwithin the cross-linked shell molecule.

Specifically, 0.2 grams of crosslinked modified DAB dendrimer reactionproducts obtained from Example Nos. 6, 7, 8, 9, 10, 11, 12, 13, 15, 16and 17, above, were reacted with 1.2 molar aqueous hydrochloric acid in10 milliliters of chloroform. The reaction mixture was stirredovernight. The aqueous layer was extracted twice with chloroform. Thevolatiles were removed under pressure to leave the free core dendrimer.The organic layer was washed twice with water and once with saturatedaqueous NaHCO₃. The organic layer contained the cross-linked shellproduct.

Based upon the foregoing disclosure, it should now be apparent that asupramolecular structures, including structures comprising a coremolecule encapsulated within a cross-linked shell molecule and hollowcross-linked shell molecules, can be produced from modification of coredendrimer molecules. The selection of the core dendrimer, reactivemonomers, crosslinking agents, crosslinking techniques and reactionconditions can be determined by one having ordinary skill in the artwithout departing from the spirit of the invention herein disclosed anddescribed. Thus, the scope of the invention shall include allmodification and variations that may fall within the scope of theattached claims.

We claim:
 1. A supramolecular structure comprising: a multi-generationdendrimer comprising a core, a plurality of interior generationsspherically disposed around the core and an outermost generationcomprising a plurality of dendritic branches having terminal groupssufficiently reactive to undergo addition or substitution reactions; andat least one cross-linkable moiety bonded to the terminal groups of eachdendritic branch via a labile bond; wherein the cross-linkable moietiesof adjacent dendritic branches are intramolecularly cross-linked to forma dendrimer having an intramolecularly cross-linked peripheral surface,wherein the core dendrimer contains catalytic centers.
 2. Thesupramolecular structure of claim 1, wherein the dendrimer is selectedfrom the group consisting of poly(propylenimine) (DAB) andpolyamidoamine (PAMAM) dendrimers.
 3. The supramolecular structure ofclaim 1, wherein the labile bond is selected from the group consistingof silicon-oxygen, silicon-oxygen-carbon, oxygen-nitrogen,nitrogen-silicon, nitrogen-carbonyl-nitrogen, silicon-acetylene, amide,blocked isocyanates and ureas.
 4. The supramolecular structure of claim3, wherein the labile bond is a nitrogen-silicon bond.
 5. Thesupramolecular structure of claim 1, wherein the dendritic branches areintramolecularly crosslinked by one method selected from groupconsisting of hydrosilation, olefin metathesis, radical polymerization,polycondensation, anionic polymerization, cationic polymerization andcoordination polymerization.
 6. The supramolecular structure of claim 5,wherein the crosslinking method is hydrosilation.
 7. The supramolecularstructure of claim 5, wherein the dendritic branches are crosslinkedwith a crosslinking agent.
 8. The supramolecular structure of claim 7,wherein the crosslinking agent is a double crosslinking agent or amultiple crosslinking agent.
 9. The supramolecular structure of claim 8,wherein the multiple crosslinking agent is selected from the groupconsisting of CH₃Si(CH₂CH₂Si(CH₃)₂H)₃; CH₃(CH₂SiH₂)₂CH₃; HC(Si(R)₂H)₃;Si(R¹)₂H₂; (SiR¹HO)₄; linear polymers selected from the group consistingof (CH₃)₃Si—O—(SiR²H—O)_(n)—Si(CH₃)₃,H(CH₃)₂Si—O—(SiPh(—OSi(CH₃)₂H)—O)_(n)—Si(CH₃)₂H,(CH₃)₃Si—O—(Si(CH₃)H—O)_(m)—(Si(CH₃)(C₈H₁₇)—O)_(n) —Si(CH₃)₃, andH₂R³Si(SiR³H)_(n)—SiR³H₂; cyclic compounds; a dendrimer; and mixturesthereof; wherein R₁ is selected from hydrogen and organic groups havingfrom about 1 to about 15 carbon atoms; R² selected from methyl and ethylgroups; R³ is selected from aryl and alkyl groups having from about 1 toabout 15 carbon atoms; n is a positive integer from about 10 to about100; and m is a positive integer from about 10 to about
 100. 10. Thesupramolecular structure of claim 8, wherein the double crosslinkingagent is of the general formula (I):

wherein where R₁ is selected from the group consisting of hydrogen ororganic groups having from about 1 to about carbon atoms, R₂ is selectedfrom the group consisting of hydrogen and organic groups having fromabout 1 to about 30 carbon atoms, and x is an integer from about 1 toabout
 4. 11. The supramolecular structure of claim 5, wherein olefinmetathesis includes the use of a ring opening metathesis polymerization(ROMP) catalyst.
 12. The supramolecular structure of claim 5, whereinolefin metathesis includes the use of a cyclic diene metathesis (ADMET)catalyst.
 13. The supramolecular structure of claim 5, wherein thecoordination polymerization is Ziegler Natta polymerization.
 14. Thesupramolecular structure produced by claim 1, wherein the core dendrimercontains metallocores.